Phase shift keying (PSK) is a known method of data modulation wherein the phase of a carrier (to represent, for example, a binary code) is set at either 0.degree. or 180.degree. within a time frame. Demodulation of a PSK signal involves the comparison of the signal with a phase reference, to enable recovery of the phase shifted carrier signals. PSK encoding requires, by definition, the presence of a coherent reference signal at the receiver to enable the decoding operation to occur.
Differential phase-shift keying (DPSK) eliminates the need for the coherent reference signal by using, as the comparison signal, the phase modulated carrier itself, which has been delayed by one symbol (or bit) time. To send a "0" value, the signal carrier is advanced in phase by 180.degree. from the signal carrier phase in the previous symbol time. To send the digital value "1", the phase of the signal carrier waveform is unchanged. The receiver is equipped with a storage facility so that it can measure the relative phase difference between the waveforms received during two successive bit intervals. Thus, the phase difference between the two waveforms, received over two successive bit intervals, enables the decoding of the signal stream and avoids the need for the presence of a coherent reference signal. DPSK encoding is widely used in time division multiple access (TDMA) systems wherein DPSK sequences are introduced in the carrier during succeeding time frames.
FIG. 1 illustrates the waveforms found in a DPSK encoded signal stream. If it is assumed that a "0" binary value is represented by a 180.degree. phase shift, then signal waveform 1, during time frames 1 and 2, represents a "1" bit and the phase shifted waveforms during time frames 3 and 3 are "0" bits. Note that delayed signal 2 lags signal 1 by one time frame. Thereafter, signal 1 and delayed signal 2 are summed so that output signal 3 exhibits a null signal value during each time frame wherein a phase reversal has occurred.
Typical bandwidths of commercially available single mode fibers, fiber amplifiers and femtosecond lasers make it possible, in principle, to achieve terabit-per-second optical fiber communications. In Ser. No. 08/758,437 to Warren et al., assigned the same Assignee as this Application, an optical communication scheme is described wherein a laser outputs a series of multi-femtosecond optical pulses. That optical pulse is applied to a grating which separates the optical pulse into its spectral components (e.g., approximately 1,000 resolvable frequency ranges or "frequency slots"). The resulting diverse frequency slots are then amplitude modulated by passing each spectral component through an acousto-optic modulator to achieve an amplitude modulation in accordance with applied data signals. The data signals from multiple transmitters are multiplexed, using a time division technique, and demultiplexing is accomplished by applying the amplitude modulated signals to an asymmetric optical demultiplexer, such as that shown in U.S. Pat. No. 5,493,433 to Prucnal et al., assigned to the same Assignee as this Application.
An important feature of the wavelength division transmission scheme described by Warren et al. is that each of the spectral components into which the incident femtosecond laser pulse is separated, is time coherent with respect to all other spectral components. The coherence of all the spectral components is due to their derivation from a common optical pulse. As will be understood from the description of the invention below, it is this coherence property that enables implementation of a demodulation scheme to enable data recovery from phase-modulated optical frequency slots.
The use by Warren et al. of an amplitude modulated signal causes a average loss of approximately 50% of the optical energy in an average signal stream. It is known that phase modulation does not result in such a penalty.
Accordingly, it is an object of this invention to provide an improved wavelength diversity, spread spectrum optical data communications system wherein DPSK modulation is utilized.
It is another object of this invention to provide a spread-spectrum optical communication system wherein demodulation apparatus for detecting DPSK encoded data makes use of known interferometric structures.